The present invention concerns microwave amplifiers intended for communication systems having a low harmonic distortion ratio when delivering an output power close to the maximum power.
In communications, the allowed level of intermodulation is 50 dB below the level of the information signal. Therefore the ratio between the output power and the maximum power of the amplifiers is at present limited to a very low value, for example one-tenth, in order to avoid intermodulation which could not thereafter be eliminated by means of conventional filters. In order to increase this ratio, it is known to use feedforward control.
In the following, the intermodulation due to the harmonics, the greater part of which is due to the 3rd harmonic, will be called "distortion".
U.S. Pat. 3,471,798, applied for on Dec. 26, 1967 by Bell Telephone Laboratories discloses a feed forward amplifier. FIG. 1a, reproduces FIG. 1 of this patent, shows three directional couplers (power dividers) 10, 17 and 15 as well as an error injection network 20, two elemental amplifiers 11 and 12, two delay compensating network 14 and 18 and two further adjustable phase shifters 23 and 24. The operation of the circuit is fully explained in the specification FIG. 1b showing a very similar circuit is the FIG. 1 of the article entitled "A wideband feedforward amplifier" which appeared in the December 1974, issue of "IEEE Journal of Solid-State Circuit", volume SC-9, No. 6, pages 422 to 428. This second low-distortion amplification circuit differs from the first by the presence of an attenuator between the couplers C.sub.2 and C.sub.3 and by the omission of the phase shifter 24.